The invention relates to flow measurement and in particular to a venturi type device for sensing flow by measuring a differential in static pressures.
The total pressure of a flowing fluid is comprised of a velocity pressure component and a static pressure component. When the velocity is increased there is a reduction in the static pressure. The well known theory of a venturi flow meter takes advantage of this principle. The flow area within the duct is reduced, resulting in an increased velocity, and the area thereafter expanded back to the original size. Static pressures are measured upstream of the meter and at the throat or reduced section of the meter. The differential static pressure can be used to calculate the flow rate, provided the density of the fluid flowing is known.
The reduction in flow area followed by the expansion results in some non-recoverable pressure loss, with this loss being a function of the angle of convergence, the angle of divergence and the amount of the restriction. If there is very little area reduction, it is difficult to get sufficient pressure differential to appropriately sense the flow. If on the other hand the area reduction is excessive, non-recoverable pressure losses will be high since they are a function of the high velocities occurring. A flow area in the throat which is 25% of the flow area in the duct is normal although this may range from 9% to 64%. This area reduction is often represented as a beta factor with beta being defined as the square root of the area of the throat divided by the area of the duct. The corresponding beta factors vary from 0.3 to 0.8. For a conventional venturi having a beta factor of 0.5, a convergence angle of 20 degrees (10.degree. from the axis on each side) and a divergent angle of 15 degrees (71/2.degree. degrees from the axis on each side) the non-recoverable pressure loss is approximately 15 to 18% of the measured pressure differential. The venturi requires a length of about 4.0 duct diameters in addition to a minimum upstream straight length of 2 duct diameters.
If the flow approaching a venturi is swirling this swirling flow is increased as the diameter is restricted. This increased velocity further reduces the static pressure beyond what would normally be expected, thereby introducing error into the measurements.
The conventional venturi effects the area reduction by reducing the outer bounding surface or periphery.
U.S. Pat. No. 1,143,631 shows a differential pressure fluid measuring device wherein an insert is placed within a straight pipe to reduce the flow area obtaining the high velocity in an annular throat.
Proper operation of a venturi requires a uniform temperature and flow condition at the approach. It accordingly requires considerable length of straight duct upstream of the venturi to give the flow conditions time to become uniform after the last flow perturbation. In some cases insufficient length is available because of physical limitations. Inlet ducts carrying air flow to a cold pulverizer are often located in cramped quarters and include within the duct system introductions of hot and cold air into a mixed stream. This results in temperature unbalance across the duct and the cramped quarters restrict the ability to find a sufficient length of straight duct into which to install a conventional venturi.
It is accordingly desirable to have a venturi type device of shorter length than the standard venturi and one which is more tolerant of unbalanced flow or temperature conditions.